Structure and Function of the Blood in Cats

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What is Blood?

Blood is the life-maintaining transport fluid that circulates oxygen and nutrients throughout the body, carries away waste products, and helps defend against disease. Blood consists of numerous components, such as cells and proteins, that circulate in a fluid called plasma. The millions of cells that circulate in blood include erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). Besides transporting vital components, the blood plays an important role in the immune functions of the body and is vitally important to coagulation (ability of blood to clot properly).

Various components of blood are produced in several tissues and organs. Red blood cells, platelets and some white blood cells are manufactured in the bone marrow, which is present in the central cavity of the long bones of the body. Other white blood cells are formed in the lymph nodes. Proteins that circulate in the blood are manufactured in the liver and by circulating white blood cells. At times the spleen is also a site of the production of some blood components.

Where is Blood Located?

Blood is located in almost every part of the body, because it circulates through the body's heart, arteries, veins and capillaries (the tiny vessels that connect arteries and veins). Any tissue containing blood vessels normally contains blood.

What is the General Structure of Blood?

Blood consists of four main parts: red blood cells, white blood cells, platelets and plasma.

Red blood cells (RBCs). These are the most numerous cells found in the blood. In the normal cat, there may be 6 to 10 million RBCs in a microliter of blood. RBCs are disc-shaped cells that contain hemoglobin, an important protein that transports oxygen. Mature red blood cells are unique in that they do not contain a nucleus. (The nucleus is the small, oval area in the cell that contains DNA genetic material.)

White blood cells. There are several classes of white blood cells (WBCs) that circulate in the blood. At any given time, in the normal cat, there are roughly 5,000 to 19,000 of these cells per microliter of blood. The types of white blood cells are as follows:

  • Neutrophils are the most numerous of all white cells. In the normal cat there are usually 2,500 to 12,500 of these cells per microliter of blood. They form a primary defense against bacterial infections. They move out of blood vessels into infected or inflamed tissue in order to attack the infection or injury.

  • Lymphocytes are an active component of the immune system and are manufactured in the bone marrow, lymph nodes, spleen and other lymphatic tissues. In the normal cat, approximately 1500 to 7000 of these cells are present in each microliter of blood. A major function of many lymphocytes is to produce antibodies.

  • Monocytes circulate in the blood until they are needed in tissues that are inflamed or infected. They then leave the blood and enter such tissues where they mature into cells called macrophages. Macrophages are capable of engulfing and destroying harmful organisms and other materials. At any one time, there are usually 0 to 850 monocytes present in each microliter of blood in the normal cat.

  • Eosinophils play an important role in the response of the body to allergic and inflammatory reactions, and to parasitic infestations. In the normal cat, only about 0 to 750 cells are seen per microliter of blood, but their numbers may be dramatically increased if parasites or other foreign protein are present in the body.

  • Basophils are the rarest of all white blood cells and are not usually seen in blood samples. They participate in many of the same reactions that eosinophils are involved in. Finding basophils in the circulating blood is significant.

    Platelets. Platelets are not cells; they are very tiny disks that look like flat plates. They are produced primarily in the bone marrow. Their major function is to plug any leak that develops in the walls of blood vessels and to start the process of blood clotting. In normal cats, there are often 200,000 to 400,000 platelets per microliter of blood.

    Plasma. Plasma is the fluid portion of blood. It is a watery mixture of proteins, minerals and other chemicals vital to the body. It contains substances such as sodium, potassium, chloride, magnesium, glucose (blood sugar), and various enzymes that are produced in the liver, pancreas, kidneys, and muscles. Plasma also contains important clotting factors and many body waste products.

    What are the Functions of Blood?

    Each component of blood has very specialized and important functions.

  • Red blood cells contain hemoglobin, which is a red, iron-rich protein. Hemoglobin enables red blood cells to carry oxygen from the lungs to all parts of the body. Red blood cells give blood its color. When the blood is rich in oxygen it is red, and when there is little oxygen in the blood, the blood is blue. Because blood traveling from the lungs to the body usually contains lots of oxygen, blood in the arteries is normally red. Much of the oxygen is removed from the small capillaries by the body tissues, so blood in the veins tends to be blue in color.

  • The white blood cells defend the body against disease. They destroy bacteria and foreign material, they stimulate inflammation and assist in the healing process, and they produce proteins called antibodies that destroy bacteria, viruses, and other diseases. WBCs move in and out of the blood stream, depending upon where they are needed.

  • Platelets help the blood to clot. They group together to form clumps, plugging any holes that develop in blood vessels. Clumps of platelets form a scaffolding upon which a blood clot may form. Formation of a blood clot is a complicated process called coagulation.

  • Plasma is the watery material that carries all other components of the blood within the blood vessels. If water is lost through dehydration, wounds and burns, then the blood can become thickened, almost like sludge, and circulation will be adversely affected.

  • What are the Common Diseases of Blood?

    There are several very important and sometimes life-threatening diseases of the blood. Diseases of the blood generally involve either too many or too few of a particular cell or blood component.

  • Red blood cells. Some disorders involving the red blood cells include anemia and polycythemia:

    With anemia there are fewer circulating RBCs than normal. Because anemia decreases oxygen delivery to cells, affected individuals are often tired or weak. Their gums may be pale as well. There are numerous causes of anemia. Anemia may arise because red blood cells are not produced in adequate numbers, because they are lost from the blood stream, or because they are destroyed.

    Polycythemia is the presence of too many RBCs in the blood. This condition is extremely rare in the cat.

  • White blood cells. The most common disorders involving the white blood cells are generally associated with infections, including bacterial, viral, fungal and parasitic infections. In response to these conditions, the body usually manufactures lots of a particular type of white blood cell, so cell numbers in the blood can become very elevated. White blood cell numbers can also become elevated when cancer in the bone marrow causes the production of cancerous white blood cells. The presence of these abnormal white blood cells in the circulation is called leukemia.

  • Platelets. The most common disorder associated with platelets is thrombocytopenia, which is a decrease in the number of circulating platelets. The number of platelets can fall if there is not enough being produced in the bone marrow, if they are being consumed as quickly as they are produced, or if they are lost from the body through continued bleeding.

  • Plasma. The most common disorder involving plasma is a decrease in the circulating proteins. Protein levels may fall if the liver does not produce enough of the protein, albumin, or if protein is lost from the blood or body. Plasma proteins are partially responsible for holding water in the blood vessels. When protein levels fall below critical level water leaves the blood stream and enters the tissues or cavities of the body, causing edema.

    What Types of Diagnostic Tests are Used to Evaluate Blood?

  • A complete blood count (CBC) measures the size, number and maturity of both the red and white blood cells within a specific blood sample. Alterations in the CBC may indicate the presence of either minor or serious disease processes. A decrease in the red blood cell count indicates anemia. Increases in the red cell count might suggest polycythemia. Increased white blood cells may indicate inflammation, bacterial infections or other infections. Severe elevations in WBCs are seen with leukemias. A decreased WBC count may occur with some viral infections or with overwhelming bacterial infections.

  • A platelet count assesses the number of platelets in the blood.

  • A serum biochemistry profile measures many components of the serum in the blood. Serum is the watery portion of the blood that remains after plasma has been allowed to clot. Biochemistry tests detect various blood proteins, the amount of sodium, potassium and chloride in the blood, blood sugar, and numerous enzymes present in the circulation.

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